Fix clang undefined version error for ssh token

We use common libcryptsetup-token.sym version script that contain
all symbols, but some of them are optional.

As clang linker treats missing symbols as errors, the linker
phase for ssh token fails as optional cryptsetup_token_buffer_free
is not defined.
(Most of distros has this option still disabled, though).

As the sym file is also example for token authors, removing symbols
there is not an option. For clang, we can use --undefined-version option,
but it is not supported by other linkers, so it requires non-trivial
checks for usable LDFLAGS (for both autoconf and meson).

Instead, fix it by simply defining the symbol in ssh token, which
duplicates the internal libcryptsetup functionality.

Fixes: #830

tokens/libcryptsetup-token.sym

@@ -1,9 +1,13 @@
 CRYPTSETUP_TOKEN_1.0 {
-    global: cryptsetup_token_open;
-	    cryptsetup_token_open_pin;
-	    cryptsetup_token_buffer_free;
-	    cryptsetup_token_validate;
-	    cryptsetup_token_dump;
-	    cryptsetup_token_version;
-    local: *;
+	global:
+		/* Mandatory functions */
+		cryptsetup_token_open;
+		cryptsetup_token_version;
+
+		/* Optional functions */
+		cryptsetup_token_open_pin;
+		cryptsetup_token_buffer_free;
+		cryptsetup_token_validate;
+		cryptsetup_token_dump;
+	local: *;
 };

tokens/ssh/libcryptsetup-token-ssh.c

@@ -39,13 +39,20 @@ int cryptsetup_token_open(struct crypt_device *cd, int token,
 	char **password, size_t *password_len, void *usrptr);
 void cryptsetup_token_dump(struct crypt_device *cd, const char *json);
 int cryptsetup_token_validate(struct crypt_device *cd, const char *json);
-
+void cryptsetup_token_buffer_free(void *buffer, size_t buffer_len);
 
 const char *cryptsetup_token_version(void)
 {
 	return TOKEN_VERSION_MAJOR "." TOKEN_VERSION_MINOR;
 }
 
+void cryptsetup_token_buffer_free(void *buffer, size_t buffer_len)
+{
+	/* libcryptsetup API call */
+	crypt_safe_memzero(buffer, buffer_len);
+	free(buffer);
+}
+
 static json_object *get_token_jobj(struct crypt_device *cd, int token)
 {
 	const char *json_slot;